RC continuous-time filters are utilized in various applications, such as in communications systems. As with other types of filters, RC continuous-time filters may be designed to selectively filter out the parts of a signal that have frequencies above or below a desired cutoff frequency. Typically, RC continuous-time filters are utilized in applications where the signals are expected to remain continuous in time and have analog levels. Since continuous-time filters can typically be utilized without the need for signal sampling, continuous-time filters provide a significant operating-speed advantage over the switched-capacitor filter counterparts.
One example of a popular application in which RC continuous-time filters are utilized is digital subscriber line (“DSL”) communications systems. DSL communications systems have been introduced and implemented by communications systems providers in recent years to provide customers with a wide variety of interactive multi-media communications signals over existing plain old telephone system (“POTS”) communications lines. As shown in FIG. 1, and known in the art, a typical DSL communications system 100 includes several basic components. It is noted that FIG. 1 merely presents a simplified representation of a typical DSL communications system that is sufficient for the purpose of this discussion.
As shown in FIG. 1, a typical DSL communications system 100 includes an analog front end (“AFE”) 102, a hybrid 104, a back-matching resistor 106, a scaling transformer 108, one or more DSL lines 110, and one or more connecting communications system lines 112. An AFE 102 is a primary interface component between a DSL communications system and other types of communications systems, such as T-carrier (e.g., T1, T3) or optical-carrier (e.g., OC-1, OC-48) communications systems. Although not shown in FIG. 1, a complete DSL communications system typically includes two or more of the DSL systems 100 interconnected via one or more DSL lines 110.
Within an AFE of a DSL communications system, such as that depicted in FIG. 1, one or more RC continuous-time filters may be utilized. Typically, the components of a DSL system, with the exception of some portions of the communications lines, are contained in a common location. As a result of this practice, DSL system components, including RC continuous-time filters integrated within an AFE, may be subject to temperature variations affected by ambient temperature variations as well as by variations in operating conditions (e.g., signal strengths or operating durations). For example, in typical applications of DSL communications systems, operating temperatures of system components may vary from −40° C. to 125° C. due to ambient temperature and/or operating condition variations. It is noted that the preceding discussion, with respect to DSL communications systems, merely presents one example of the utilization of RC continuous-time filters, and there are many other applications in which RC continuous-time filters are or may be utilized and in which temperature variation may be a concern.
Although RC continuous-time filters offer an operating-speed advantage, as discussed above, these filters typically require some means of tuning in order to set and maintain a desired cutoff frequency. A major reason that tuning of RC continuous-time filters is necessary is because the cutoff frequency of the filter is dependent on the values of the resistance and capacitance elements of the filter, and these values will typically vary due to temperature variations. For example, in a filter built with “High-resitivity Poly0” resistors and “Interpoly” capacitors (note, these component types are known in the art and have certain temperature variation characteristics), the variation in cutoff frequency may vary by as much as 0.15%/° C. In contrast, an acceptable range of cutoff frequency variation, depending on the application, is typically less than 0.04%/° C. It is noted that in an RC continuous-time filter that is built with High-resitivity Poly0 resistors and Interpoly capacitors, the temperature variation of the RC component of the filter, and thus the cutoff frequency of the filter, is typically dominated by the temperature variation of the resistors which can be as large as 0.15%/° C.
Therefore, there is a need for a system and method for a filter tuner for tuning RC continuous-time filters in order to set and maintain cutoff frequencies within acceptable tolerances with regard to temperature variations.